The invention relates to a method for cleaning a particulate filter for pollution control of exhaust gases from a motor vehicle engine.
The exhaust gases of diesel engines which use gas oil as fuel contain both gaseous pollutants such as unburned hydrocarbons and nitric oxides or carbon monoxides, and solid pollutants which are chiefly made up of soot particles. The pollution control standards applying to diesel engines necessitate increasingly advanced elimination of soot particle emissions in the exhaust gases of these engines.
In order to ensure the elimination of soot particles, it is necessary to post-treat the exhaust gases using a particulate filter.
Such a particulate filter is installed in the engine""s exhaust line and has at least one filtration element constituted by a filtering support with a porous structure, if appropriate associated with another pollution control element, fixed in a metal casing which is connected to the exhaust line. The filtering support or supports disposed in the metal casing of the filter, which are known as xe2x80x9ccanningxe2x80x9d, may be constituted by porous ceramic elements. The exhaust gases pass through the filtering support between a filter inlet end and a filter outlet end, thereby enabling the solid particles of soot to be retained in suspension in the exhaust gases, so as to obtain a purified gas on the output side. The particulate filter may be arranged downstream of a catalyser.
In the course of the engine""s operation, the filtering support becomes laden with soot particles which deposit themselves in the filtering support, with the result that the filter becomes progressively clogged up. Unclogging the filter can be done by burning the soots deposited in the filtering support, along that same exhaust line, whilst the motor vehicle is in use.
In the presence of oxygen, the soots burn at temperatures of the order of 550xc2x0 C. to 600xc2x0 C. The exhaust gases of a diesel engine on a touring vehicle only seldom reach such thermic levels. Accordingly, it is necessary to encourage the filtration elements to begin regenerating by adding a fuel additive that enables the combustion temperature of the soots to be lowered.
The additives may be made up of mineral compounds in solution in an organic solvent which, mixed in certain proportions into the gas oil, follow the gas oil circuit. They are thus injected via the injection system into the combustion chamber, and their combustion residues are contained in the exhaust gases.
The presence of these additives in the particulate filter where they are intimately mixed with the soot particles allows them to act as a catalyser during combustion of the soot particles and to lower the soot ignition temperatures to approximately 350xc2x0 C. to 550xc2x0 C.
The combustion of soots in the particulate filterxe2x80x94catalysed if appropriatexe2x80x94makes it possible to remove the organic compounds and the carbon contained in these soots. However, once the filter has finished regenerating by combustion, mineral residues are left behind which are retained in the filtering support. These mineral residues are in particular made up of residues derived from additives that aid soot combustion, other additives incorporated in the gas oil, together with a lubricant; these residues may also be made up of particles caused by engine wear or coming from the environment outside, when such particles are not retained by the engine""s air filter. All these residues remain stored in the filtering support of the particulate filter. From the aspect of chemical composition, many types may be present in these residues, for example oxides, sulphates, nitrates or phosphates of elements such as cerium, zinc, calcium, copper, iron or nickel.
The particulate filter progressively becomes choked and, after operating for a certain time in the exhaust line, the clogging of the filtering support may be such that it results in a deterioration in engine performance and a rise in fuel consumption. In order to restore satisfactory filter operating conditions it is then necessary to either replace the filter with a new filter or clean the filter.
Given the high cost of particulate filters, it may be financially beneficial to avoid having to replace the filter.
French patent applications 9907682 and 0105582 have proposed methods for cleaning a particulate filter that involve passing through the filtering support of the particulate filter, in the direction running from the outlet end to the inlet end of the filter, a stream of cleaning fluid capable of dissolving or detaching the residues deposited in the channels or pores of the filtering support of the particulate filter. In order to improve the cleaning action involved in these methods, a pressurised gas such as air capable of flushing through the cleaning fluid and the detached or dissolved residues may be put through the filter.
It is also possible to perform rinsing phases by passing water through the filter, and hot-air drying phases.
These methods, which make it possible to restore the particulate filters on motor vehicles after they have been in use for some time, necessitate relatively complex and costly installations which incorporate means for injecting fluid or gas through the particulate filters and means for automatically controlling the different successive phases of the cleaning operation.
The cleaning operations must be performed in specialised workshops or factories fitted out with cleaning installations which must be used to regenerate large numbers of particulate filters, thereby justifying the high cost of these installations.
Furthermore, in the case of certain particularly tenacious residues it is very difficult, within a limited period, to make the residues dissolve or loosen sufficiently.
It is therefore the object of this invention to propose a method for cleaning a particulate filter that has a filtering support with a porous structure, associated with a catalyser if appropriate, fixed in a metal casing having an inlet port and an outlet port, when in service the casing of the filter being joined to an upstream section of an exhaust line of a motor vehicle by its inlet port and to a downstream section thereof by its outlet port, so that passing through the filter, between the inlet port and the outlet port, is a stream of exhaust gases from the motor vehicle""s engine containing particles of soot, of which at least a proportion that is retained by the filtering support undergoes combustion by being heated up in the filtering support, said combustion being catalysed if appropriate, so as to regenerate the filter in the exhaust line, the cleaning process making it possible to facilitate the removal of mineral residues that are clogging up the filter following regeneration of the particulate filter by combustion and to remove some or all of the residues, without the use of a complex and costly installation.
To this end:
at least part of the exhaust line containing the particulate filter is disassembled in order to separate it from the motor vehicle; and
the filtering support is contacted with a cleaning solution for a period of time sufficient to ensure that the filtering support is soaked, so as to ensure or facilitate the detachment of the residues retained in the filtering support.
The method according to the invention may be implemented in accordance with one of the following modalities:
at least part of the filter containing the filtering support is introduced into a cleaning solution and the filtering support is kept in the cleaning solution to soak it;
the filtering support is introduced into and held in the cleaning solution in a position such that an axis of the filter and of the filtering support pointing in the direction in which the gases circulate between the inlet port and the outlet port of the filter is substantially vertical, and in a direction such that an exhaust gas inlet end is located beneath an outlet end in the filtering support;
the filtering support is introduced into and held in the cleaning solution in a position such that the axis of the filter and of the filtering support pointing in the direction in which the gases circulate between the inlet port and the outlet port is substantially horizontal, soaking of the filter being done with the filter""s outlet port obturated;
the cleaning fluid is introduced inside the filter casing and soaking of the filtering support is done inside the filter casing;
in a second cleaning phase, a stream of cleaning fluid is passed through the filtering support of the particulate filter, between an outlet end and an inlet end;
the cleaning fluid is maintained at room temperature while the filtering support of the filter is being soaked;
the cleaning fluid is heated to a temperature substantially higher than room temperature while the filtering support of the filter is being soaked;
during the cleaning operation consisting in soaking the filtering support of the filter, the cleaning fluid is agitated by one of the following means: mechanical agitation, ultrasonic agitation and circulating the cleaning solution by means of a pump;
displacement or agitation of the particulate filter is produced during the cleaning-by-soaking operation, in order to facilitate the detachment of residues deposited in the filtering support of the filter;
the part of the filter that includes the filtering support is introduced into the cleaning solution contained in a cleaning tank, a remaining part of the filter that includes the outlet port of the filter casing being sited above the level of the cleaning solution in the soaking tank, and the level of the cleaning solution in the tank is maintained at a substantially constant position;
the outlet port of the filter casing is hermetically sealed following immersion of the filtering support in the cleaning solution, thereby preventing the cleaning solution from passing through the outlet port of the filter casing;
at the end of the soaking operation to clean the filtering support of the particulate filter, the filtering support of the particulate filter is subjected to rinsing and drying;
processing and regeneration of the cleaning solution containing the particulate filter cleaning residues, either dissolved or in suspension, is carried out;
after disassembling the part of the exhaust line that contains the particulate filter, regeneration of the filtering support of the particulate filter is carried out by combustion of the soots present in the filtering support using one of the following methods:
i) holding the particulate filter in an oven at a temperature ensuring combustion of the soots,
ii) passing a hot gas such as air through the filtering support of the particulate filter, so as to ensure combustion of the soots present in the filtering support; and
iii) thereafter placing the filtering support of the particulate filter in contact with the cleaning solution;
the filter is regenerated on the exhaust line of the motor vehicle, prior to disassembling the part of the exhaust line that contains the particulate filter.
The invention also relates to an installation for cleaning particulate filters.
The cleaning of a particulate filter in accordance with the present invention may be carried out in a phase prior to a cleaning operation performed in accordance with one of the methods outlined in French patent applications 9907682 and 0105582; soaking the filtering support makes it easier to detach or dissolve the residues. Alternatively, soaking may be followed solely by an operation involving rinsing and drying the filtering support.